Two-component developers used in electrophotography are composed of a toner and a carrier. The carrier is a carrier material which is mixed and stirred with the toner in a developer box to impart a desired charge to the toner, and carries the charged toner to electrostatic latent images on a photoreceptor to form toner images. The carrier, also after forming the toner images, is held on a magnet, remains on a development roll, further again returns to the developer box, is again mixed and stirred with new toner particles, and is repeatedly used in a certain period.
The two-component developers, different from one-component developers, are ones in which a carrier stirs toner particles and imparts a desired chargeability to the toner particles while having a function of transporting the toner, thus having good controllability in developer design. Therefore, the two-component developers are widely used especially in the fields of full-color machines requiring high-quality images, and high-speed machines requiring reliability and durability of image sustainability.
For obtaining high-quality images in these two-component electrophotographic developers, ferrite particles such as a Cu—Zn ferrite or Ni—Zn ferrite are used as a carrier in place of an oxide-filmed iron powder and a resin-coated iron powder. Ferrite carriers using these ferrite particles have many advantageous characteristics to obtain high-quality images, such as generally spherical and controllable in magnetic properties, over conventional iron powder carriers. Further, resin-coated ferrite carriers in which the ferrite particles as a core material are coated with various resins are improved in wear resistance, durability, etc., and controllable in volume specific resistance.
However, since the ferrite is a ceramic, it has a drawback of smashing by impact though having a high hardness after the ferrite reaction. In the sintering step in production where the ferrite reaction is made to occur, gaps between particles become small especially with decreasing particle size, and particles themselves fuse by heating in a high temperature, thereby becoming difficult to maintain a spherical shape.
In resent years, in such two-component electrophotographic developers, the high-speed and full-color imaging of the development performance has strongly been demanded. For obtaining a high-quality image output in such a demand, a problem of making carriers and toners of small particle sizes arises.
Regarding toners, various toners having small particle sizes and sharp particle distributions by polymer toner technologies, etc., have been proposed.
On the other hand, making a carrier of a small particle size, that is, use of small particle-sized ferrite particles makes a formed magnetic brush soft, and makes the specific surface area of the carrier large and the held toner amount large, resulting in anticipation of large effects on image qualities such as the image density, fogging in image, toner scattering and gradation.
However, making ferrite carriers of small particle sizes raises a problem, in the production steps, of making it difficult to maintain a spherical shape of the ferrite particles as described above. Although for improving the wear resistance and the durability, the surface of the carrier core material (ferrite particle) is coated with various kinds of resins as described above, if the shape of the ferrite particles is impaired, the coating nonuniformity and exposed parts of the core material are generated at the time of resin-coating. Thus, the carrier performance is not fully achieved, and the high-quality image and the elongated life (high durability) required for developers are not accomplished.
In the production steps of ferrite particles, when particles are shredded in the shredding step after sintering, if fused particles are shredded by strong impacts, they are thoroughly crushed, and amorphous particles come to mingle. Amorphous particles are difficult to remove, so if the resin-coating is performed with the amorphous particles in the next step, the image quality is adversely affected due to uniform coating not being formed on the amorphous particles, interfering with fluidity, etc.
Although for maintaining a spherical shape, fusion between particles is prevented by lowering the sintering temperature, the carrier core material becomes porous, and in the resin-coating step for the carrier core material surface, the resin penetrates inside, thereby being liable to cause variations in carrier performances.
Sintering to form ferrites conventionally involves filling raw materials in a sagger of alumina, etc., and sintering in a tunnel-type sintering furnace. However, with a small particle size, since fusion between particles is easily generated, the sintering temperature cannot be too much raised, thereby causing variations in the surface property. This results in an obstacle to the uniform coating formation in the next resin-coating step, and leads to the performance deterioration.
The technology to produce ferrite particles having a spherical shape, a uniform surface property and a small particle size has not been sufficient as described above. For achieving the high-quality and the elongated life when a two-component developer is prepared with a toner, various attempts have been made to provide a ferrite carrier having a small particle size, a spherical shape and a uniform surface property.
Patent Document 1 (Japanese Patent Laid-Open No. 07-98521) describes a carrier for electrophotography having a 50% average particle size (D50) of 15 to 45 μm, a specified particle distribution and a definite ratio of specific surface areas by different measuring methods.
Patent Document 2 (Japanese Patent Laid-Open No. 2001-117285) describes a carrier for developing electrostatic charge images which uses nucleus particles (carrier core material) having a volume average particle size of 25 to 50 μm and a volume resistance and a shape index within definite ranges, and which has a coating layer containing electroconductive particles formed on the nucleus particle surface.
Patent Document 3 (Japanese Patent Laid-Open No. 08-292607) describes a two-component developer wherein a coating layer composed of a resin material is formed on the surface of carrier core material particles, and wherein the shape indexes of the carrier core material particles and the carrier particles after resin-coating are specified, and the former shape index is constituted to be larger than the latter shape index.
Patent Document 4 (Japanese Patent Laid-Open No. 09-197722) describes a carrier for developing electrostatic charge images obtained by forming a coating layer on nucleus particles (carrier core material) which have a saturation magnetization of 50 to 70 Am2/kg, an average particle size of 30 to 40 μm, a weight ratio of not more than 22 μm of 2.0 to 17.0 wt %, and a specified shape index.
Patent Document 5 (Japanese Patent Laid-Open No. 02-255539) describes a process for producing a ferrite powder comprising a wet mixing step for raw powders, an atomizing step to adjust the particle size to 10 μm to 100 μm, and a stirring and sintering step at 1,100° C. to 1,200° C. in this order to obtain ferrite powder. It contends that in this production process, the production steps are simplified, and since the obtained ferrite powder is of a spherical shape, and has a small specific surface area in comparison with amorphous powders, the improvement in the dispersibility and the fluidity are achieved.
Although the inventions according to the Patent Documents 1 to 4 described above make the ferrite carrier core materials of small particle sizes, specify the shape indexes, the specific surface area, etc., and provide mainly spherical ferrite core materials, a carrier core material which has a small particle size, and yet a high sphericity and surface uniformity, and a low standard deviation, a resin-coated ferrite carrier using the carrier core material, and a process for producing the same, are not obtained. The Patent Document 5 describes a simplified process for producing a ferrite powder, and only shows that the obtained ferrite powder is of a spherical shape.
[Patent Document 1]: Japanese Patent Laid-Open No. 07-98521
[Patent Document 2]: Japanese Patent Laid-Open No. 2001-117285
[Patent Document 3]: Japanese Patent Laid-Open No. 08-292607
[Patent Document 4]: Japanese Patent Laid-Open No. 09-197722
[Patent Document 5]: Japanese Patent Laid-Open No. 02-255539
Accordingly, an object of the present invention is to provide a resin-coated ferrite carrier using a carrier core material having a small particle size, a high sphericity and surface uniformity, and a low standard deviation, a process for producing the same, and an electrophotographic developer using the resin-coated ferrite carrier and having high-quality images and an excellent durability.